US lawmakers prodded by the frozen food industry have moved to protect schools' ability to count pizza sauce as a vegetable in lunches for students.

In an annual spending bill covering the US Department of Agriculture, which has oversight over subsidised school meals, a joint House-Senate panel voted to prevent the agency from restricting pizza, hot chips, and starchy vegetables.

A Republican summary of the legislation was unveiled on Monday and may be approved this week.

The report cheered the defeat of "overly burdensome and costly regulations" and hailed "greater flexibility for local school districts."

The American Frozen Food Institute industry lobby group hailed the measure, which it said "recognises the significant amounts of potassium, fibre and vitamins A and C provided by tomato paste and ensures that students may continue to enjoy healthy meals such as pizza and pasta."

But Margo Wootan, director of nutrition policy at the Centre for Science in the Public Interest, said the legislation was about protecting pizza makers, not nutrition.

"Pepperoni pizza is not a vegetable," Ms Wootan said.

Ms Wootan said existing rules defined a full serving of vegetables as eight tablespoons, except for a "loophole" that set the amount of tomato paste required at two tablespoons, roughly what goes on a slice.

The USDA had proposed early this year to require eight tablespoons of tomato paste in one vegetable serving and limit school lunches to two servings per week of french fries or other starchy vegetables.

"The Congress basically stepped in to protect industry's ability to continue to sell two of the most unhealthy foods in the school lunch program: pizza and french fries," Ms Wootan said.